How to slim down a cuboid?
I would make the heart shaped base, then make sure to Ctrl +  A to apply scale and rotation. Then select the surface and inset  I. You can type in the exact distance in the pop up on the lower left of the screen.
If you want to copy it to create a second object, press Shift +  D,  P, then Selected.
You can extrude that face to make a smaller heart.